The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a polishing pad having a grooved pattern for a chemical mechanical polishing apparatus.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar outer surface presents a problem for the integrated circuit manufacturer. Therefore, there is a need to periodically planarize the substrate surface to provide a flat surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is then placed against a rotating polishing pad. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. In addition, the carrier head may rotate to provide additional motion between the substrate and polishing surface.
A polishing slurry, including an abrasive and at least one chemically-reactive agent, may be supplied to the polishing pad to provide an abrasive chemical solution at the interface between the pad and the substrate. CMP is a fairly complex process, and it differs from simple wet sanding. In a CMP process, the reactive agent in the slurry reacts with the outer surface of the substrate to form reactive sites. The interaction of the polishing pad and abrasive particles with the reactive sites on the substrate results in polishing of the substrate.
An effective CMP process not only provides a high polishing rate, but also provides a substrate surface which is finished (lacks small-scale roughness) and flat (lacks large-scale topography). The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad. The polishing rate sets the time needed to polish a layer. Because inadequate flatness and finish can create defective substrates, the selection of a polishing pad and slurry combination is usually dictated by the required finish and flatness. Given these constraints, the polishing time needed to achieve the required finish and flatness sets the maximum throughput of the CMP apparatus.
A recurring problem in CMP is non-uniformity of the polishing rate across the surface of the substrate. One source of this non-uniformity is the so-called xe2x80x9cedge-effectxe2x80x9d, i.e., the tendency for the substrate edge to be polished at a different rate than the center of the substrate. Another source of non-uniformity is termed the xe2x80x9ccenter slow effectxe2x80x9d, which is the tendency of center of the substrate to be underpolished. These non-uniform polishing effects reduce the overall flatness of the substrate and the substrate area suitable for integrated circuit fabrication, thus decreasing the process yield.
Another problem relates to slurry distribution. As indicated above, the CMP process is fairly complex, requiring the interaction of the polishing pad, abrasive particles and reactive agent with the substrate to obtain the desired polishing results. Accordingly, ineffective slurry distribution across the polishing pad surface provides less than optimal polishing results. Polishing pads used in the past have included perforations about the pad. These perforations, when filled, distribute slurry in their respective local regions as the polishing pad is compressed. This method of slurry distribution has limited effectiveness, since each perforation in effect acts independently. Thus, some of the perforations may have too little slurry, while others may have too much slurry. Furthermore, there is no way to directly channel the excess slurry to where it is most needed.
Another problem is xe2x80x9cglazingxe2x80x9d of the polishing pad. Glazing occurs when the polishing pad is heated and compressed in regions where the substrate is pressed against the pad. The peaks of the polishing pad are pressed down and the pits are filled up, so the polishing pad surface becomes smoother and less abrasive. As a result, the polishing time increases. Therefore, the polishing pad surface must be periodically returned to an abrasive condition, or xe2x80x9cconditionedxe2x80x9d, to maintain a high throughput.
In addition, during the conditioning process, waste materials produced by conditioning the pad may fill or clog the perforations in the pad. Perforations clogged with such waste materials do not hold slurry effectively, thereby reducing the effectiveness of the polishing process.
An additional problem associated with filled or clogged pad perforations relates to the separation of the polishing pad from the substrate after polishing has been completed. The polishing process produces a high degree of surface tension between the pad and the substrate. The perforations decrease the surface tension by reducing the contact area between the pad and the substrate. However, as the perforations become filled or clogged with waste material, the surface tension increases, making it more difficult to separate the pad and the substrate. As such, the substrate is more likely to be damaged during the separation process.
Yet another problem in CMP is referred to as the xe2x80x9cplanarizing effectxe2x80x9d. Ideally, a polishing pad only polishes peaks in the topography of the substrate. After a certain period of polishing, the areas of these peaks will eventually be level with the valleys, resulting in a substantially planar surface. However, if a substrate is subjected to the xe2x80x9cplanarizing effectxe2x80x9d, the peaks and valleys will be polished simultaneously. The xe2x80x9cplanarizing effectxe2x80x9d results from the compressible nature of the polishing pad in response to point loading. In particular, if the polishing pad is too flexible, it will deform and contact a large surface area of the substrate, including both the peaks and the valleys in the substrate surface.
Accordingly, it would be useful to provide a CMP apparatus which ameliorates some, if not all, of these problems.
In one aspect, the invention is directed to a polishing pad for polishing a substrate in a chemical mechanical polishing apparatus. The polishing pad comprises a first polishing region having a first plurality of substantially circular concentric grooves with a first width and a first pitch, and a second polishing region surrounding the first polishing region and having a second plurality of substantially circular concentric grooves with a second width and a second pitch. At least one of the second width and second pitch differs from the first width and first pitch.
In another aspect, the polishing pad comprises a polishing surface having a first polishing region and a second polishing region surrounding the first polishing region, a spiral groove formed in the polishing surface, the spiral groove having a first pitch in the first polishing region and a second, different pitch in the second polishing region.
In another aspect, the polishing pad comprises a first polishing region having a first plurality of substantially circular concentric grooves, and a second polishing region surrounding the first polishing region and having a plurality of substantially serpentine grooves.
In another aspect, the polishing pad comprises a first polishing region having a first plurality of substantially circular concentric grooves, and a second polishing region surrounding the first polishing region and having a second plurality of substantially circular concentric grooves. A center of the second plurality of concentric grooves is offset from a center of the first plurality of concentric grooves.
In another aspect, the polishing pad comprises a first polishing region having a first plurality of substantially circular concentric grooves, and a second polishing region surrounding the first polishing region and having a plurality of groove arc segments. The groove arc segments are disposed along concentric circular paths such that each groove arc segment does not radially overlap a groove arc segment on an adjacent path.
In another aspect, the polishing pad comprises a first polishing region having a first plurality of substantially circular concentric grooves, and a second polishing region surrounding the first polishing region and having a spiral groove.
Implementations of the invention may include the following. Each groove may have a depth of at least about 0.02 inches, a width of at least about 0.015 inches, and a pitch of at least about 0.09 inches. A third polishing region may surround the second polishing region and have substantially circular concentric grooves. The width and pitch of the grooves in the third region may be equal to the width and pitch of the grooves in the first region. The pitch of the groove or grooves in the first region may be different, e.g., larger, than the pitch of the groove or grooves in the second region. The width of the groove or grooves in the first region may be different, e.g., smaller, than the pitch of the groove or grooves in the second region. Specifically, the first pitch may be about two times larger than the second pitch, and the second width may be about six times greater than the first width. The grooves in the first region may cover about 25% of the surface area of the first region, and the grooves in the second region may cover about 50% of the surface area of the second region. The spiral groove may have a uniform width. The serpentine grooves may have a pitch between about one and two times their amplitude, or between about one-and-one-half and two times their width. The grooves in the second region may have a width of about 0.125 inches and a pitch of about 0.2 inches. The serpentine groove may have an amplitude between about 0.2 and 0.4 inches. The center of the first plurality of circular grooves may be offset from the center of the second plurality of circular grooves by a distance approximately equal to a pitch of the second plurality of grooves. The grooves in the third region may be concentric with the grooves in the first region.
Advantages of the invention include the following. The polishing pad provides improved polishing uniformity. The grooves of the polishing pad provide an effective way to distribute slurry across the pad. The grooves are sufficiently wide that waste material produced by the conditioning process can be flushed from the grooves. The polishing pad is sufficiently rigid to avoid the xe2x80x9cplanarizing effectxe2x80x9d. The polishing pad""s relatively deep grooves also improve the pad lifetime.
Other features and advantages will be apparent from the following description, including the drawings and claims.